Liposomes are vesicles composed of a bilayer of lipid molecules, much like natural cellular membranes, and have been developed to deliver pharmaceutical agents. Pharmaceutical agents, including those not normally taken up by cells because they are too large to pass through membrane channels, or are degraded in plasma may be delivered by encapsulating the agent within a liposome. It is believed that due to their structural similarity with membranes, liposomes and their contents are taken up by cells. Unfortunately, cellular uptake of liposomes is to a great extent by an endocytotic mechanism, resulting in the liposomes being engulfed in endosome or lysosome organelles only to be degraded and/or expelled as waste along with their pharmaceutical contents. Consequently, an often insufficient amount of the pharmaceutical agent ever becomes present in the cellular cytoplasm or nucleus where it is active.
In an attempt to address the problems associated with endocytosis, liposomes comprising cationic lipids, sometimes referred to as “fusogenic” liposomes, have been developed. These liposomes fuse with cellular and intracellular membranes due to electronic attraction between negatively charged phospholipids of membranes and the positively charged liposome (Wrobel et al, Biochimica et Biophysica Acta, 1995, 1235:296). Thus when fusion of liposome and membrane occurs, the contents of the liposome is expelled across the membrane. An example of such a liposome is lipofectin which comprises the cationic lipid DOTMA (N-(1-(2,3-dioleoyloxy)propyl)-N,N,N-trimethylamine). Other cationic lipids having been formulated into liposomes include DOTAP (1,2-dioleoyloxy-3-(triethylammonium)-propane), DOGS (N,N-dioctacedylamidoglycylspermine) and DDAB (dimethyl-dioctadecyl-ammonium-bromide). However, a problem associated with cationic liposomes is that when administered in vivo, they tend to bind various factors such as proteins in the plasma resulting in the formation of aggregates that are too large to enter cells. Such aggregates are believed to circulate poorly in the vasculature and accumulate in lungs and possibly cause pulmonary embolisms.
It would therefore be desirable to provide lipids and liposomes that are fusogenic with membranes and thus capable of delivering pharmaceutical agents to tissues or cells yet do not have the inherent aggregation problems associated with cationic lipids and liposomes.